The German patent publication 19,853,942 C1 discloses a linear drive of this type comprising a housing defining a receiving space wherein drive means are arranged. The drive means are a component of an electric linear motor, which is inserted into the receiving space as a cartridge-like unit and is fixed in relation to the housing. The housing in this case delimits the elongated receiving space, within which the drive means are provided. By the supply of electrical energy it is possible for the armature to be moved along the longitudinal axis within the said space. A load driver is connected with the armature and a component to be moved is attached to it. The component to be moved by the linear motor or, respectively, the linear drive may for example be a guide slide of a machine tool.
The German patent publication (utility model) 20,113,014 U1 discloses an electrodynamic linear direct drive. This linear direct drive comprises a housing with a receiving space extending in the longitudinal direction of the housing. Within the housing an elongated stator in the form of a support tube is arranged, on which the annularly designed permanent magnets are arranged coaxially in sequence. In order to act as a magnetic return the elongated stator may consist of magnetically conductive material and more particularly of low retentivity material. The armature with a coil winding is arranged on this stator. On excitation of the coil the armature may be moved in relation to the stator in the longitudinal direction of the housing and by way of the load driver entrains the component which is to be moved.
For the electrical control or, respectively, regulation of the electrodynamic direct drive it is important for the position of the armature and accordingly furthermore of the component attached to the load driver, to be detected. Hitherto for this purpose a relative zero point has been set for the armature, in relation to which the absolute position of the armature is detected during operation. Such methods of measurement are slow and require the employment of additional staff, since the position of the armature must be reset or calibrated in relation to the zero point.
Furthermore so far various different measurement methods have been tried out rendering possible automatic displacement measurement of the armature in relation to the housing, the use of displacement measurement based on electromagnetic waves, such as microwaves, radar waves or the like not so far having been considered, since the coils and permanent magnets within the housing possess a magnetically conductive surface, on which the electromagnetic waves are propagated and therefore render impossible or substantially interfere with, any displacement measurement.
The patent publication WO 99/31463 describes a distance measuring device and a method for measuring the relative distance of a piston within a cylinder. The distance measuring device for example comprises a microwave sensor as a sensor, which on the basis of the reflection of the microwaves may detect the position of the piston. Unlike electrodynamic linear direct drives the surface of the cylinder and the piston constitutes a surface which cannot so influence the electromagnetic waves that accurate measurement is not feasible. The bore of the cylinder is for example defined by an electrically conductive material or a plastic so that measurement on the basis of electromagnetic radiation functions in any case. However when, as in the case of an electrodynamic direct drive, magnetically conductive surfaces play role, such displacement measurement has not been possible so far.